In the past, casing has been cemented downhole using Portland cement-based materials. The Portland cement is mixed with water or available aqueous solution and circulated to properly spot the material for properly sealing off the outside of the casing from the formation. The Portland cementing systems have generally been provided to well operators by oilfield service companies. These service companies have brought to the well site, mixing equipment and personnel so that the Portland cement can be continuously mixed with water to obtain the proper ratio of cement to water for proper solidification on the casing exterior. More recently, an alternative to Portland cement systems has been developed. This system involves the addition of blast-furnace slag (BFS) to water-based mud for slurry densities in the range of between 10-20 lbs./gal., depending on the mud weight and the requirements applicable to the specific operation. The thickening time of BFS/mud mixtures is controlled by activators and retarders. Activators accelerate hydration of BFS and reduce thickening time and improve early compressive strength development. Some of the more effective activators are alkali materials that increase the mud pH. The addition of BFS and activators to mud has negligible effect on the plastic viscosity, yield point, and gel strength over and above the properties of the original mud. The rheological properties can be further adjusted using chemical thinners or deflocculents, dilution of the mud with water, or a combination of both steps.
BFS/mud mixtures can be activated by thermal energy or chemical activation. Chemically, BFS is similar to Portland cement. It is the residue from ore and other additives developed in a molten state, along with iron in a blast furnace. BFS is discharged from a blast furnace at a temperature between 2500.degree.-2900.degree. F. and quenched as a molten slag to produce a glassy, granular material. Optimization of BFS/mud mixtures puts the solids by volume content at anywhere in the range between 16-45% and reasonably approximates the concentrations of equivalent density Portland cement slurries.
BFS is less reactive than Portland cement. As a consequence, BFS/water-based mud mixtures or BFS/water mixtures must be chemically activated to achieve set in the downhole wellbore environment, and Portland cement/water mixtures usually must be chemically retarded. Chemical activation is simpler and less expensive than retardation. Typically, sodium hydroxide and sodium carbonate are the most widely used activators for BFS/water-based mud mixtures because they are commonly available. Sodium hydroxide has a greater impact on setting time, while sodium carbonate has a greater influence on the manner of set and the compressive strength. Above 80.degree. F., sodium carbonate is used in greater concentrations than sodium hydroxide because there is sufficient thermal energy to reduce setting times, while the higher sodium carbonate concentration aids in the early compressive strength development. In temperatures below 80.degree. F., the sodium hydroxide concentration can be equal or slightly higher than sodium carbonate concentration because of low thermal energy available to activate the slag hydration. Typically, activator concentration in the formulations varies between 2-24 lbs./barrel of slurry for most formulations, depending on the temperature and the amount of retarding material present. Yet another advantage of using BFS/mud mixtures is uniform compressive strength build-up over time in situations where significant temperature differentials exist between top and bottomhole temperatures.
When using BFS/mud mixtures in the field, the BFS is typically mixed with the mud in the mud pits on the rig. Batch mixing in rig pits or mud premix tanks or cement mixers is possible. In some instances, this has required isolation of mud to be used for batch mixing with the BFS. Testing is then separately done on samples of the mixture. Upon completion of the testing, activators, dispersants or other additives were added to the isolated fluid. This was usually done prior to cementing. Use of this procedure involved several operational considerations which could adversely affect the rig equipment and/or the success of the primary cement job. The possible problems all arise from the potential for premature set of the slag-mix slurry, if all of the components are added to the mix mud before the cementing operation is initiated. Generally, according to API procedures, BFS is less abrasive than barite and, therefore, undue wear on pumps and other equipment is not a major problem. In the past, however, concerns about early set-up of the BFS/mud mixture in the surface equipment has been an impediment to successful cementing, especially where there are substantial thermal differences between ambient temperatures at the surface, where the activator is added, and bottomhole temperature, where the activator is to perform. Normally, BFS/mud mixtures were designed to have sufficient thickening time at the downhole temperature to allow slurry placement. In many situations where the downhole temperature exceeds the surface temperature, the mixture thickening time is longer on the surface. This leaves ample time to flush the lines and other surface equipment of residual fluid before it sets. However, in deepwater applications, the slurry surface temperature may be higher than the bottomhole circulating temperature. Therefore, slurries designed at the surface for lower bottomhole temperatures will set faster on the surface. In the past, this has required a dedicated line from the mixing tank to the rig pumps to minimize contamination of surface equipment. Additionally, due to flow variability upon pumping of the BFS/cement slurry, fine tuning of the amount of activator was necessary to account for variations in flowrates, as well as any bottomhole temperature variations. If the batch was premixed in the mud pits, there would be no opportunity to control the rate of addition of activator to the batch blend. Additionally, batch mixing presented uniformity problems as well as the potential that the batch could solidify if pumping problems developed.
In the past, radio tracer injection techniques have been used to facilitate foot-by-foot measurements of Portland cement coverage behind a casing. The principle used was injection of a uniform tracer material with a short half-life to allow measurement of cement placement, mud displacement, and the mixing that takes place at displacement fronts. The injection technique for the radioactive material is illustrated in FIG. 1 of IADC/SPE Paper 14778 entitled "Evaluation of Cementing Practices by Quantitative Radio Tracer Measurements," authored by Kline, et al., delivered in 1986 at the IADC/SPE Conference in Dallas, Tex.
The system illustrated in FIG. 1 has many advantages over the prior techniques which have been used to add activator to the BFS/mud slurry. The apparatus and method illustrated in FIG. 1 deal with the possible risks to rig equipment and the success of the primary cement job, for which no answers were available in the referenced 1993 article by Cowan, the inventor of U.S. Pat. No. 5,058,679, which is discussed in the detailed description below. One of the concerns to the well operator is if attempts are made to continuously mix the activator with the BFS/mud slurry, the operator was required to call out a cementing service company who would bring, at great expense, continuous mixing equipment previously used for Portland cement-based systems to conduct the inline mixing of the BFS with the mud. In order to save rig time and expense, ideas began to develop about using the rig equipment for mixing the BFS with the mud. The economic incentive was to avoid the cost associated with hiring a service company for the mixing operation and to minimize rig time due to delays which could ensue from such an operation with a cementing service company. However, as pointed out by Cowan, the risks of batch mixing in the rig pits and adding a precise amount of activator for the expected downhole conditions created certain risks. For example, where the subsurface temperatures were significantly lower than the surface temperatures, additional activator would have to be added to allow for the lower temperature downhole. However, this would shorten the set-up time for the BFS/mud slurry and create operational problems if the slurry, once activated, was not quickly placed downhole where the expected lower temperature would be encountered. Those skilled in the art appreciated that the pumping of the BFS/mud slurry is not a continuous operation at a smooth flowrate. Upon initial presentation into the casing internals, the mud "free falls" until the casing internals are filled and the mud starts its progress outside of the casing adjacent the formation. At that point, fluid losses could occur due to washouts or high porous segments, as well as resistance to flow can occur, all of which act to put additional resistance on the surface pumping equipment, which in turn induces the rig personnel to alter the operation of the surface pumps to avoid exceeding a predetermined pressure. This results in a slowing down of the flowrate, which in turn can cause problems if a large batch has been mixed in the rig equipment and activator already added. Continuous addition of activator on a real-time basis to reflect the actual operating flowrates directly compensates for flow fluctuations while at the same time minimizing the time between activator injection and final placement. In essence, as soon as the activator is added, the mud is pumped through the cementing head and toward its final destination. If for any reason there is a flow interruption, the surface equipment is essentially free of activated BFS/mud slurry.
Accordingly, the apparatus and method of the present invention have been developed to improve on the systems for activator addition to BFS/mud mixtures. The present invention allows for sensitivity to changing flowrates and pressure conditions downhole during the placement of the BFS/mud mixture. The method of addition also minimizes the risk of line plugging when using surface mud circulating equipment. The activator concentration can be adjusted on a real-time basis and precisely added for the expected downhole temperatures to be encountered, while at the same time minimizing the risk of fouling surface equipment or the inside of the casing.